1. Field of Invention
The invention relates to an accelerometer and, in particular to a capacitance accelerometer.
2. Related Art
An accelerometer is used to measure the acceleration caused by an external force. For example, it can be applied in a vehicle automatic safety system for collecting the information such as the kinetic energy of the vehicle or the external force applied on the vehicle. Due to the development of micro electromechanical system (MEMS), the semiconductor technology may integrate the mechanical devices and circuits to manufacture the accelerometer. The accelerometer manufactured by the semiconductor technology has the advantages of low cost, decreased size and weight, and enhanced reliability.
According to the different measuring methods, the accelerometers include the piezoresistive accelerometers, the capacitance accelerometers, and the piezoelectric accelerometers. Herein, the capacitance accelerometers calculate the acceleration according to the capacitance variation. Regarding to the structure, the capacitance accelerometer may provide an out-of plane measuring mechanism or an in plane measuring mechanism. The out-of plane measuring mechanism utilizes large area planer electrode plates for measuring, and the in plane measuring mechanism utilizes comb electrodes, which are arranged interdigitatedly, for measuring.
Hereinafter, a conventional capacitance accelerometer 1 using the in plane measuring mechanism will be described. As shown in FIG. 1, the conventional capacitance accelerometer 1 includes a proof mass 10, an elastic part 11, a first comb electrode 12, and a second comb electrode 13. The proof mass 10 is connected with a fixing block 14 by the elastic part 11. If there is no external force, the proof mass 10 stays at a static position. In this case, the first comb electrode 12 is connected with the proof mass 10, and the second comb electrode 13 is disposed with respect to the first comb electrode 12 and located at another fixing block 15. When the external force is applied, the proof mass 10 moves up and down along the direction of arrow D. In this case, the distance between the first comb electrode 12 and the second comb electrode 13 changes, and then the capacitance variation between the electrodes 12 and 13 is measured for estimating the corresponding acceleration.
To increase the sensitivity of the capacitance accelerometer 1, the numbers of the comb electrodes 12 and 13 are increased, or the distance between the first comb electrode 12 and the second comb electrode 13 is decreased. However, if the numbers of the comb electrodes 12 and 13 are increased, the entire area of the accelerometer 1 must be increased, which is undesired for the trend of product minimization. In addition, the present manufacturing machines have their limitations while decreasing the distance between the electrodes 12 and 13.
Therefore, it is an important subject of the invention to provide an accelerometer having high sensitivity and small size.